Studies of plastic-ablator compressibility for direct-drive inertial confinement fusion on OMEGA

The compression of planar plastic targets was studied with x-ray radiography in the range of laser intensities of I approximately 0.5 to 1.5x10(15) W/cm2 using square (low-compression) and shaped (high-compression) pulses. Two-dimensional simulations with the radiative hydrocode DRACO show good agreement with measurements at laser intensities up to I approximately 10(15) W/cm2. These results provide the first experimental evidence for low-entropy, adiabatic compression of plastic shells in the laser intensity regime relevant to direct-drive inertial confinement fusion. A density reduction near the end of the drive at a high intensity of I approximately 1.5x10(15) W/cm2 has been correlated with the hard x-ray signal caused by hot electrons from two-plasmon-decay instability.     

Rayleigh-Taylor growth stabilization in direct-drive plastic targets at laser intensities of approximately 1 x 10(15) W/cm2

Direct-drive, planar-target Rayleigh-Taylor growth experiments were performed for the first time to test fundamental physics in hydrocodes at peak drive intensities of ignition designs. The unstable modulation growth at a drive intensity of approximately 1 x 10(15) W/cm2 was strongly stabilized compared to the growth at an intensity of approximately 5 x 10(14) W/cm2. The experiments demonstrate that standard simulations based on a local model of electron thermal transport break down at peak intensities of ignition designs (although they work well at lower intensities). The preheating effects by nonlocal electron transport and hot electrons were identified as some of the stabilizing mechanisms.     

Ionen- und lonenpaar-Beiträge zur Polymerisation von Tetrahydrofuran

Ohne Zusammenfassung     

The lattice sites of carbon and hydrogen incorporated in GaAs grown by MOVPE revealed by infrared spectroscopy

Infrared absorption due to the localized vibrational modes in carbon doped GaAs and Al_0.02Ga_0.98As has been investigated and classified for C_As, C_As-Al_Ga nearest neighbour pairs and H-C_As passivated pairs. The assignment of a line at 563 cm^-1 is still unclear: it could be due to C_Ga donors or a mode of H-C_As pairs. There is no evidence for the presence of interstitial carbon.     

Interionic potentials and force constant models for rocksalt structure crystals—II

A lattice dynamical model for rocksalt structure crystals is presented in which the interionic force constants are taken to be central and the departure from the Cauchy relations is accounted for by isotropic and anisotropic deformations of the anions. The model is applied to NaBr, NaI and KBr for which C₁₂ > C₄₄ and it is appropriate to consider only the anisotropic deformations. The model parameters are determined entirely from macroscopic data. By assuming an exponential form for the repulsive part of the interionic potential the cohesive energy of the crystals may be determined from the interionic force constants. The cohesive energies are in surprisingly good agreement with experiment although the potential parameters differ considerably from those of other workers. While the dispersion curves obtained from the model are in markedly worse agreement with experiment than those from the standard breathing shell model it is shown that the potentials(in addition to giving good cohesive energies) result in a reasonably consistent dynamical model.     

Comparison of the molecular cluster model with the phonon model for Jahn—Teller active impurities in crystals

For comparing the molecular cluster model with the phonon model in describing the Jahn-Teller interaction for magnetic impurities in crystals, the Jahn-Teller energy and the Ham reduction factors are expressed in terms of the phonon Green's function of the host crystal for an orbital triplet coupled to E_g modes of vibrations. Numerical results for the case of MgO lattice have been obtained using the phonon Green's function derived from the breathing shell model.     

High-rhoR implosions for fast-ignition fuel assembly

Thick, 40 microm plastic shells filled with 25-35 atm of D2 or D3He were imploded on a low-adiabat (alpha approximately 1.3) and with a low-implosion velocity ( approximately 2 x 10(70 cm/s) on the OMEGA laser to generate massive cores of compressed plasma with high areal densities optimal for fast ignition. The targets are driven by 20-kJ relaxation adiabat-shaping laser pulses to keep the inner portion of the shell nearly Fermi degenerate. The measured kinetic energy downshift of proton spectra is in good agreement with the theoretical predictions yielding burn-averaged areal densities of 0.130+/-0.017 g/cm2 and peak rhoR during the burn of about 0.24+/-0.018 g/cm2, the largest rhoR measured on OMEGA to date. The same implosions with empty plastic shells are expected to reach 1.3 g/cm2 across the core (i.e., 2rhoR) enough to stop fast electrons with energies up to 4.5 MeV typical of fast ignition scenarios.     

Measuring E and B fields in laser-produced plasmas with monoenergetic proton radiography

Electromagnetic (E/B) fields generated by the interaction with plasmas of long-pulse, low-intensity laser beams relevant to inertial confinement fusion have been measured for the first time using novel monoenergetic proton radiography methods. High-resolution, time-gated radiography images of a plastic foil driven by a 10(14) W/cm(2) laser implied B fields of approximately 0.5 MG and E fields of approximately 1.5 x 10(8) V/m. Simulations of these experiments with LASNEX+LSP have been performed and are in overall (though not exact) agreement with the data both for field strengths and for spatial distributions; this is the first direct experimental test of the laser-generated B-field package in LASNEX. The experiments also demonstrated that laser phase plates substantially reduce medium-scale chaotic field structure.